Great attention has been paid to the effective use of various kinds of slags, which are by-products in the steel industry. In the steel industry, slags are produced as by-products, with different compositions and properties depending upon kinds of processes and facilities, and also upon kinds of steel produced by melting.
For example, blast furnace slag is produced as by-product from a blast furnace which is used in a process for producing pig iron. Furthermore, molten iron pretreatment slag, converter slag, and electric furnace slag are respectively produced as by-products from a molten pretreatment facility, a converter, and an electric furnace.
Furthermore, there are granulated slag and slowly cooled slag in the blast furnace slag, and there are desilication slag, dephosphorization slag, desulurization slag, and decarburization slag in the molten iron pretreatment slag. Even in the electric furnace slag, there are oxidizing period slag, and reducing period slag.
With respect to the kind of steel, there are plain carbon steel, super low carbon steel, special alloy steel, and stainless steel.
Of the above-mentioned slags, blast furnace granulated slag produced as by-product from the blast furnace is used as a material for use in concrete admixture, base course and others. Furthermore, it is reported that converter slag, when subjected to a treatment such as deferrization, can be used as a material for base course.
However, many of the slags other than blast furnace granulated slag have not yet found any effective application. Furthermore, with respect to a slag which has already found some applications, the situation is such that it cannot be said that presently it is sufficiently reused, because each slag has a significantly different composition and physical properties, depending upon its manufacture and its lot, and therefore it is difficult to expand the range of its reuses, and even if it can be used for base course only, the demand for it is limited.
There is no effective use for steelmaking slag produced from steelmaking process, so that presently steelmaking slag is discarded as industrial waste. The term “steelmaking slag” used in the present invention specifically means electric furnace reducing period slag, molten iron pretreatment slag, stainless slag, and converter slag, but does not include granulated blast furnace slag and slowly cooled blast furnace slag.
Some of these slags contain a β-2CaO.SiO2 phase, and others contain a γ-2CaO.SiO2 phase. Slags which contain the β-2CaO.SiO2 phase exhibit hydraulicity, so that the use thereof as a material for a cement admixture and others is now being studied. However, slags containing the γ-2CaO.SiO2 phase have not found any effective use.
This is due to a dusting phenomenon. The steelmaking slag comprises dicalcium silicate (2CaO.SiO2) as a main compound, so that in the course of a cooling process for the slag, 2CaO.SiO2 is transformed from an α phase which is a high temperature phase to a α′ phase, and then to a β or γ phase which is a low temperature phase. When 2CaO.SiO2 is transformed from the α′ phase to the γ phase which is a low temperature phase, it swells with a significant change in the density thereof and is pulverized. This phenomenon is called “dusting”.
Due to the above-mentioned dusting phenomenon, the steelmaking slag, unlike other slags, cannot be obtained in either in a massive form or in the form of particles, and therefore cannot be used as a material either for base course or for aggregate.
Conventionally, as a method of preventing the dusting caused by 2CaO.SiO2, there has been proposed, for example, a method of stabilizing 2CaO.SiO2 in the β phase with the addition thereto of a crystal stabilizer such as a boron compound (JP-A-62-162657). However, the boron compound itself is expensive and some improvements on the facilities and processes therefor are required, so that this method is costly.
On the other hand, there are known a special cement prepared by pulverizing an electric furnace reducing period slag without adding a crystal stabilizer thereto, and mixing the same with calcium aluminate 12CaO.7Al2O3 and gypsum (JP-B-62-47827), and a special cement prepared by mixing a solid solution of calcium aluminate 12CaO.7Al2O3 and CaF2 with gypsum (JP-B-62-50428 and others).
This invention is made by directing attention to the fact that although the electric furnace reducing period slag comprises non-hydraulic γ-2CaO.SiO2 as a main component, the slag also contains a large amount of 12CaO.7Al2O3 which has high hydration activity, and attempts to obtain a hardened material with a desired strength, with the formation of ettringite by the addition of gypsum.
However, hardened materials obtained from the above-mentioned special cements have poor resistance to carbonation caused by carbon dioxide gas in the air, so that it is not expected that such hardened materials have the same durability as that of a hardened material obtained from Portland cement. Furthermore, it is difficult to secure fluidity and a usable time period unless a setting retarder is used in combination. Furthermore, in the above-mentioned invention, nothing is mentioned about the mixing of the electric furnace reducing period slag with Portland cement and with tricalcium silicate 3CaO.SiO2 which is a main component of Portland cement, thereby imparting a function of providing a carbonation suppressing effect and others.
The inventors of the present invention paid their attention to the above-mentioned slag comprising γ-2CaO.SiO2 as a main component, and studied the application thereof to a cement admixture. Furthermore, they studied as to how to cope with new international standards based on the European Standards (EN Standards), and also studied the subjects of the control of heat of hydration and the prevention of carbonation.
At present, new international standards are under study abroad, which use the European Standards (EN Standards) as the basic idea thereof, and can select a cement material group largely classified based on the strength thereof in accordance with the desired objective.
According to the European Standards (EN Standards), compressive strength is broadly classified into a 32.5 N/mm2 class, a 42.5 N/mm2 class, and a 52.5 N/mm2 class (Koji Goto, Shunsuke Hanehara, Internationalization of Cement Standards—Outline and Direction of European Standards—, Cement. Concrete, No. 631, pp 1 to 8, 1999).
On the other hand, in Japan, the quality of cements has been designed based on JIS. As a result, cements with good strength revelation have been evaluated as good cements under the standardized specifications.
As a result, when classified in accordance with the EN Standard, in Japan, there are only cements which correspond to either the 42.5 N/mm2 class or the 52.5 N/mm2 class in terms of compressive strength. Therefore, at present, even if there is carried out design of mix for a concrete not having so high a design strength, the strength tends to become excessive in many cases.
The prevention of the excessive strength is important in view of the prevention of the resulting generation of excessive heat of hydration, and also in view of the prevention of the cracking after hardening by minimizing the degree of shrinkage before and after hardening.
There can be conceived a design of mix for a concrete not having so high a design strength by use of a cement with excellent strength revelation, thereby reducing the unit cement amount. In this case, however, the unit cement amount becomes extremely small, so that a problem occurs that there is formed a concrete from which ingredients are apt to be separated, having a large breeding ratio, that is, a gradients-separated concrete.
When a concrete structure is built by use of such a concrete, there is a problem that it is difficult to built a concrete structure having durability since macroscopic defects are apt to occur.
Thus, the EN Standards are characterized in that there is provided a cement of the 32.5 N/mm2 class in terms of compressive strength, which facilitates the design of mix for a concrete with not so high a design strength.
At present, a limestone-mixed cement is in a main stream of cements that are in conformity with the EN standards. The limestone-mixed cement is composed of Portland cement and a large quantity of limestone fine powder, which is capable of attaining both the prevention of excessive strength and the improvement on the resistance against the separation of ingredients of the cement.
The limestone fine powder can be regarded as an inactive powder in view of the revelation of strength, but can advantageously impart only the resistance against the separation of ingredients to the cement, thereby suppressing the revelation of excessive strength and the generation of heat of hydration. In such circumstances, studies on the limestone-mixed cement have now been actively made in Japan.
However, limestone is an important raw material in many industries. Limestone is one of precious natural resources in Japan which has scarce natural resources. If it is used only for mixing it with concrete, it will resultantly be used up, and therefore it is earnestly desired to use limestone more effectively as an industrial raw material. The limestone-mixed cement has a shortcoming that it is easily carbonated.